1. Field of the Invention
The present invention relates to a four-cycle engine in which a pair of intake valve ports is openable/closable by respective intake valves, and a pair of exhaust valve ports is openable/closable by respective exhaust valves. The pair of intake valve ports and the pair of exhaust valve ports are provided in a cylinder head of the engine such that they are positioned on both sides of a first virtual plane which contains the axial line of a cylinder bore and passes through an approximately central portion of a combustion chamber. Furthermore, a pair of ignition plugs are mounted in the cylinder head.
2. Description of Background Art
A four-cycle engine of the above type has been known, for example, from Japanese Utility Model Publication No. Hei 3-6826.
In the above-described four-cycle engine, a pair of ignition plugs is disposed in an outer peripheral portion of a combustion chamber. Alternatively, one of a pair of ignition plugs is disposed at the center of a combustion chamber and the other is disposed in an outer peripheral portion of the combustion chamber. Accordingly, in the case where both of the ignition plugs are disposed in the outer peripheral portion of the combustion chamber, a flame propagation distance in the combustion chamber due to accidental firing of either of the ignition plugs increases. Therefore, a flame propagation condition is degraded. In the case where one of the ignition plugs is disposed at the center of the combustion chamber, the flame propagation distance in the combustion chamber due to accidental firing of the ignition plug also increases. Therefore, the flame propagation condition is also degraded.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a four-cycle engine capable of minimizing the degradation of the flame propagation condition, even if accidental firing of either of a pair of ignition plugs occurs.
To achieve the above object, according to a first aspect of the present invention, a four-cycle engine includes a pair of intake valve ports openable/closable by respective intake valves and a pair of exhaust valve ports openable/closable by respective exhaust valves. The pair of intake valves and the pair of exhaust valves are provided in a cylinder head such that they are positioned on both sides of a first virtual plane which contains the axial line of a cylinder bore and passes through an approximately central portion of a combustion chamber. Furthermore, a pair of ignition plugs are mounted in the cylinder head. The pair of ignition plugs are disposed substantially symmetrically with respect to a second virtual plane which passes through the center of the combustion chamber and is perpendicular to the first virtual plane. The pair of ignition plugs is mounted in the cylinder head such that they have axial lines extending substantially parallel to the first virtual plane. The pair of ignition plugs is tilted with a distance therebetween becoming smaller toward the combustion chamber. Furthermore, end portions of the ignition plugs, which project into the combustion chamber, are disposed in a region surrounded by the intake valve ports and the exhaust valve ports.
With this configuration, the ends of the pair of ignition plugs which project into the combustion chamber, are disposed in proximity to an approximately central portion in the combustion chamber. Accordingly, it is possible to ideally propagate flame in the combustion chamber. Furthermore, even if accidental firing of either of the ignition plugs occurs, the other ignition plug is located at the approximately central portion of the combustion chamber. Accordingly, it is possible to minimize the deterioration of a flame propagation condition. Both of the ignition plugs are disposed, as seen from the direction perpendicular to the first virtual plane, in an approximately V-shape opened in the direction opposite to the combustion chamber. Accordingly, both of the ignition plugs can be easily mounted in the cylinder head with the ends of the ignition plugs, which project into the combustion chamber, disposed in proximity to an approximately central portion of the combustion chamber. Both of the ignition plugs are collectively disposed in the vicinity of the central portion of the combustion chamber. Accordingly, it is possible to enhance the degree of freedom of the shape of a water jacket on the cylinder head side and the degree of freedom of disposition of fastening bolts for fastening the cylinder head to the cylinder block. Accordingly, the sealing performance between the cylinder head and the cylinder block as well as the cooling performance can be improved.
According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, a pair of rocker shafts has axial lines extending in parallel to the first virtual plane. The pair of rocker shafts is mounted to the cylinder head such that it is positioned on both sides of the pair of ignition plugs. An intake side rocker arm corresponding to both of the intake valves is rockably supported by the rocker shaft. An exhaust side rocker arm corresponding to both of the exhaust valves is rockably supported by the rocker shaft. With this configuration, it is possible to set the width of the cylinder head at a relatively small value in the direction along the second virtual plane. Accordingly, the engine can be made more compact.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.